1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a sidelight-type spread illuminating apparatus including a light guiding plate with a light source provided on a light incident face of the light guiding plate. In the spread illuminating apparatus, planar illumination light is adapted to exit out from a light output portion of the light guiding plate.
2. Description of Related Art
In a lighting device provided with a liquid crystal display panel, a sidelight-type spread illuminating apparatus (backlight) having compact, environmentally-compatible LEDs, the LEDs being arranged along the light incident face of a light guiding plate, has been widely applied. This type of spread illuminating apparatus has particularly been used in the field of compact portable information devices such as mobile phones.
Since information devices have become much thinner in recent, further thin spread illuminating apparatuses have also strongly demanded. To accomplish the above demand, for example, the following spread illuminating apparatus has been proposed. That is, an inclined surface is provided near the light incident face of a light guiding plate as that the thickness of the light guiding plate gradually reduces from the light incident face thereof toward a light output face thereof. With this structure, the thickness of the light incident face of the light guiding plate is configured to correspond to the light output face of each LED. On the other hand, the light output face portion of the light guiding plate is allowed to have further thin configuration. See, for example, Japanese Patent Application Laid-Open No. 2007-287550.
An example of conventional spread illuminating apparatus is shown in FIG. 8. A spread illuminating apparatus 100 has a light guiding plate 101 and a LED 112. The light guiding plate 101 is formed in a manner so as to gradually reduce its thickness from the side of a light incident face 104 toward a light output face 102 by providing an inclined face 106 in the vicinity of the light incident face 104. The LED 112 is implemented on a flexible printed circuit board 110, and placed on the light incident face 104 of the light guiding plate 101.
In addition, the spread illuminating apparatus 100 has a diffusing sheet 121 and a prism sheet 122 layered to each other on the side of the light output face 102 of the light guiding plate 101, and further has a light-shielding sheet 124 around on the light incident face 104 as well as around on an end face 105 placed opposite to the light incident face 104. A reflection sheet 123 is provided on the back surface 103 of the light guiding plate 100.
Here, in the spread illuminating apparatus 100, a flexible printed circuit board 110 is bent at a bending line 111 to have a downwardly curved shape. The flexible printed circuit board 110 is thus arranged on the inclined face 106 of the light guiding plate 101. A mounting portion 110a located in the rear side of the bending line 111 is arranged substantially perpendicular to the light incident face 104 such that a light emitting face 112a of the LED 112 placed on the mounting portion 110a faces the light incident face 104 in a substantially parallel relationship. On the other hand, a bent portion 110b located in the front side of the bending line 111 is fixed to the inclined face 106 by a fixing means such as a double-sided adhesive tape.
Such a configuration where the flexible printed circuit board 110 is bent to place the bent portion 110b along the inclined face 106 of the light guiding plate 101 is an advantageous configuration for inhibiting light leakage from the inclined face 106. Moreover, optical sheets such as the diffusing sheet 121 and the prism sheet 122 are placed as that their ends at the side of the light incident face 104 are layered on the bent portion 110b of the flexible printed circuit board 110. Accordingly, even if light is leaked from the inclined portion 106 through the bent portion 110b of the flexible printed circuit board 110, the occurrence of a so-called hot spot, which locally produces a high-luminance region, can be inhibited with the optical sheets and the like.
However, in the spread illuminating apparatus 100 having such a configuration as shown in FIG. 8, when, for example, the fixing means for fixing the flexible printed circuit board 110 to the light-guiding plate 101 deteriorates due to environmental factors and the like, there has been a problem that the bent portion 110b can be peeled off from the inclined portion 106 through a reaction force of bending of the flexible printed circuit board 110. When such a peeling off occurs, the optical properties of the spread illuminating apparatus 100 may deteriorate because, for example, a so-called hot spot is produced by the increase of leaked light from the inclined face 106, and/or because brightness and the uniformity of brightness are reduced by the occurrence of the displacement of the LED 112 with respect to the light guiding plate 101.
Furthermore, in the configuration where the ends of optical sheets such as the diffusing sheet 121 and prism sheet 122 at the side of the light incident face 104 are layered on the bent portion 110b of the flexible printed circuit board 111, since these ends are placed in a state where they are raised by the thickness of the flexible printed circuit board 111 (and, the thickness of the fixing means for the inclined face 106), there has been a problem that it is likely to cause a misassembling such that the optical sheets override the upper surface (the surface at the side of the light output face 102 of the light guiding plate 101) of a housing frame 125 covering the perimeter of the light guiding plate 101 as shown in FIG. 9. FIG. 9 depicts an example where the diffusing sheet 121 and two prism sheets 122a and 122b were layered on the light output face 102 of the light guiding plate 101, and the displacement of the upper prism sheet 122b has occurred. In FIG. 9, the flexible printed circuit board 110 mounting the LED 112 is not shown.
This problem will have become more important when the housing frame 125 needs to be as thin as possible for responding to the demand of further thin spread illuminating apparatuses.